Inspection and Maintenance of Steel Girders-6

8/12/2019 Inspection and Maintenance of Steel Girders-6

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b) This is similar to welded plate girder except that theheight of girder is less and top flange plate is provided

with only 10 mm plate and shear connectors. Bottomflange plates consist of two plates with continuous sidefillet weld to resist tensile stress. Bearing stiffeners andintermediate stiffeners are the same as welded plategirders.

c) Cross frames consist of angles with rivetted connectionsas well as splice material for spans 18.3 m and aboveare provided with rivetted connection.

d) Bearings provided are sliding type bronze bearings. Oneend of the span is provided as free end in which steelbearing plate slides on bronze plate and the other end ofspan is provided with bearing plate resting on steel plate.Bearing on steel plate provides movement only duringcreep and shrinkage of concrete deck. Otherwise it actslike a fixed end.

Composite girders are designed as unpropped girders. In railwaytrack bridges, propping of girder till whole concrete is setand acts monolithically with steel beam is not possible due toheight of substructure and bed conditions. Composite girders aremostly used in new constructions and doublings. Its use posesproblems during regirdering on account of the requirementof existing substructure jacketing due to increased dead weightand difference in height, as distance between rail level andbed block is more than that of the existing steel girder of samespan.

Steel used tor fabrication of composite girder of I-sectionshould conform to IS 2062 Grade ‘B’ 1992 fully killed andnormalised. Since no rolled section is available in Indiaconforming to IS 2062, Grade ‘B’,channel shear connectors areused of steel confirming to IS 2062 Grade ‘A’ (Old IS 226). Crossframe and splice material are of steel confirming to

IS 2062 Grade ‘A’ with rivetted connections. Fig. 2.7 shows thedetails of a welded composite girder.



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2.3.2 Open web steel girders - Truss type with rivettedconstruction/welded construction

Mostly all spans of 30.5 metres and above are of openweb steel girders in railway track bridges. Open webgirders are used as (a) deck type (underslung of 30.5metres span as standard girder and above that, spans areof nonstandard design) (b) Semi through (non-standard)(c) through standard spans of 30.5, 45.7, 61.0 and 76.2metres.

Mostly open web type girders are used for track bridgesover valleys and large rivers on account of economy in cost.This is achieved due to low depth of bottom of girder from the raillevel (for through girder bridges). Presently steel used isconforming to IS 2062 Grade ‘B’ (old IS 2062-’A’) for rolledsections and rivet steel conforming to IS 1148 for spans of 30.5,45.7, 61.0 and 76.2 metres. Steel used for rail cum road/railbridges truss with rivetted construction of for spans 91.5 and

122.2 metres is conforming to IS - 8500 (old IS 961) for rolledsections. For welded floor systems such as cross girders andstringers steel is conforming to IS 2062 Grade ‘B’ and rivet steelconfirming to IS - 1149.

Nonstandard open web girders are of many types as indicated inFig. 1.5 to 1.13.

Warren truss (triangulated truss) with vertical members at everypanel point is used as standard through and deck typegirder for track bridges. Figure No.2.8 indicates types oftruss.

Design of open web girder for primary and secondarystresses is as stipulated in the Indian Railways Steel BridgeCode Clauses 3.3.1 and 3.3.2.

Clause 3.3.1 The primary stresses in the design of triangulatedstructures are defined as axial stresses in members calculatedon the assumption that -



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CONFIGURATION

6 @ 5321 = 31926

7 3 1 5

CLEAR SPAN

30.5m THROUGH

30.5m UNDER SLUNG

4 6 0 0

10 @ 3190 = 31900

7 3 1 5

47.5m THROUGH

8 @ 5905 = 47240

9 0 0 0

61.0m THROUGH8 @ 7875 = 63000

10 @ 7880 = 78800

1 0 5 0 0

76.2m THROUGH

Fig. 2.8 CONFIGURATIONS OF BROAD GAUGESTANDARD OPEN WEB GIRDERS

cL

cL

cL

cL

cL

(All dimensions are in mm)



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a) All members are straight and free to rotate at the joints.b) All joints lie at the intersection of centroidal axes of the

members.c) All loads including the weight of the members are

applied at the joints.

Clause 3.3.2 Secondary stresses - In practice the assumptionsmade in Clause 3.3.1 are not realised and consequentlymembers are subjected not only to axial stresses but also tobending and shear stresses. These stresses are defined as

secondary stresses, and fall into two groups :a) Stresses which are the result of eccentricity of

connections and of off-joint loading generally e.g. loadsrolling direct on chords, self weight of members and windloads on members.

b) Stresses which are the result of elastic deformation ofthe structure and the rigidity of the joints. These areknown as deformation stresses.

Girders are designed, fabricated and erected in such a manneras to minimise secondary stresses as far as possible.

In nonprestressed girders, deformation stresses mentioned inClause 3.3.2 (b) of Steel Bridge Code shall in the absence ofcalculations be assumed to be not less than 16.67% of the deadload and live load stresses including impact. Now with computer

aided design (CAD) it is possible to calculate all stresses by“3D” analysis with the end conditions defined.

In case of prestressed open web girders deformationstresses may be ignored, hence as per Steel Bridge Codeclause No.3.3.9 all open web girders for railway trackbridges of span 30.5 m and above shall be prestressed forachieving saving of steel due to ignoring deformation stresses.

Since the method adopted on Indian Railways is holding goodwith economical maintenance, therefore, it is continued till date.Rules for prestressing are given in Appendix “A” of Steel bridgecode. In actual sense it is predeforming of members to ensurethat under full load (i.e. Dead Load + Live load + Impact) the



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members along their centrodial axis are of nominal lengths(without any deformation).

Method of fabrication and erection for achieving prestressing in theopen web girder is as under :

I. During Fabrication :

(a) Marking and drilling of holes in the main gusset plates ofeach panel points of top and bottom chord, is to be doneon nominal layout as per details of joints indicated in

fabrication drawing so as to preserve the intersectionangles of nominal profile (uncambered profile)corresponding to fully loaded state of truss.

(b) Length of all members of truss are to be fabricated tocamber lengths as indicated in camber sheet of fabricationdrawing, with the group of connection holes at both endsdrilled through holes of main gussets which are drilled onnominal layout. For this purpose, hole drilling jig is to be

manufactured for each member with the distance betweenthe group of end holes altered by the amount of camberallowance, i.e. difference between nominal length andcamber length.

(c) Bottom chords camber length are same as nominal lengthto facilitate the fabrication of stringer.

II. During erection (assembly) at site:

The work is to be done in successive stages so as to prestress(predeform) the members of truss, that is to say members oferected truss in the unloaded state is a reverse way to whatwould have happened to them under full load if the truss weredesigned and erected without prestressing. Assembly procedurein stages to be adopted for site assembly is mentioned in para2.6. Fig. 2.22 indicates the stages of erection for prestressing.

Track structure over the stringers or top chord isprovided with the same thickness of sleeper throughout thelength of span hence track will have the same camber profile oftruss during unloaded state.



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5 9 0 5 N O M I N A L

U 2

5 9 0 5 N O M I N A L

U 3

U 4

7 3 1 6 W I T H C A M B E R

7 3 1 5 N O M I N A L

5 9 0 5 N O M I N A L

9 4 0 1

N O M I N A

L

9 4 0 3

. 5 W I T H

C A M

B E R

7 3 1 6 . 5 W I T H C A M B E R 5 9

1 0

. 5 W I T H C A M B E R

5 9 0 5 N O M I N A L

L 3

5 9 0 5 W I T H C A M B E R

5 9 0 5 N O M I N A L

9 4 0 1 N O M I N A L

9 4 0 8 . 5 W

I T H C A M

B E R

7 3 1 8 W I T H C A M B E R

7 3 1 5 N O M I N A L

9 4 1 0

N O M I N A

L

7 3 1 6 . 5 W I T H C A M B E R

9 4 0 0 W

I T H C A N B

E R

7 3 1 5 N O M I N A L

9 4

0 1 N O M I N A L

9

4 0 8 . 5 W I T H

C A M B E R

U 1

5 9 0 5 N O M I N A L

N O T E :

A l l d i m e n s

i o n s a r e i n m m

2 1

3 7

5 0

5 3

F i g . 2 . 9

C A M B E R

D I G R A

M

O F

4 5 . 7

M

S P A N

L 4

L 2

5

9 0 5 W I T H C A M B E R

L 1

5

9 0 5 W I T H C A M B E R

L 0

5 9 0

5 W I T H C A M B E R

5 9 1

0 . 5

W I T H C A M B E R

5 9 1

0 . 5

W I T H C A M B E R

7 3 1 5 N O M I N A L

5 9 0 5 N O M I N A L



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2.3.2.1 OPEN WEB THROUGH GIRDER:- STANDARD SPANCamber sheet of fabrication drawings of all open web girders

indicates the designed camber at each panel point (based on joint deflection calculated by Williot diagram) and altered camberlength as well as nominal length of each members of truss.Fig. 2.9 shows details of each member camber length andnominal length of span 45.7 metre for guidance.

Stress sheet of open web girders indicates the members,maximum force in members, nature of stress and cross section

area required and provided.Table 2.2 shows sample of a part ofstress sheet of 61.0 metre span for guidance.

All bottom chord panel points are designated as L0, L

1, L

2, L

3, L

4

etc. upto the last panel (L stands for Lower chord panel) and alltop chord panel points are designated as U

1, U

2, U

3, U

4 etc. upto

last panel (U stands for Upper chord). This facilitates identifyingeach member. L

0, L

1, L

2 are numbered in increasing direction of

kilometre. (Fig. 2.10)

In open web girders of through type, centre to centre of trussesand height of truss is governed by the schedule of movingdimensions rather than structural design requirement, hence thefloor system is essentially required for track structure support.This is consisting of cross girders and stringers (rail bearers.)The distance between rail level to bottom of girder varies from1232 to 1777 mm for spans 30.5 to 76.2 metres.

2.3.2.2 Main components of open web through spans -Rivetted construction

a) Bottom chord - Tension member designated as L0-L

1 or

L0-L

1-L

2 etc. and made up of 2 nos. rolled or built up

channels back to back placed at a distance apart andconnected with batten plates to form a member. Toincrease cross section for middle chords additional

web plate is provided with rivetted connection.b) Top chord - Compression member designated as U

1-U

2

or U1-U

2-U

3 etc. and made up of 2 nos. rolled or built up

channels and top flange plates. Channels are placed with



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OF SPAN

10 @ 3190 = 31900

(a) 30.5m SPAN UNDERSLUNG TYPE(BA - 11401)

8 @ 5905 = 47240

4 6 0 0

7 3 1 5

(b) 47.5m SPAN THROUGH TYPE(BA - 11361)

Note: All dimensions are in millimetres

Fig. 2.10 NUMBERING OF PANEL POINTS

OF SPAN

U0 U1 U2 U3 U4 U5 U6 U7 U8 U9 U10

U1

U2

U3

U4

U5

U6

U7

L1

L2

L3

L4

L5

L6

L7

L8

L9

L1

L2

L3

L4

L5

L6

L7

L8

L0

cL

cL

Inspection and Maintenance of Steel Girders-6

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